1. Field of the Disclosure
The present disclosure relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems such as Wi-Fi systems are prone to a “hidden node problem,” which arises when two stations (STAs) are within range of the same access point, but are too far apart to be aware of each other. The two STAs are therefore hidden from each other. The hidden node problem can be exacerbated by the presence of obstructions between the STAs for the access points. For example, building penetration losses are typically on the order of 11-20 dB. Consequently, an indoor STA may be hidden from an outdoor STA even though they may be physically proximate to each other. Similarly, two indoor STAs may be hidden from each other if they are separated by one or more walls, doors, or other obstructions within the building.
Transmissions from one hidden STA can interfere with transmissions from the other hidden STA without either STA recognizing that interference is occurring. When two signals collide at an access point, the access point generally detects the stronger signal and treats the weaker signal as interference. Consequently, a stronger hidden STA can overwhelm a weaker hidden STA and consume an unfair proportion of the resources of the access point, particularly when a large proportion of the resources of the access point is available for use by the STAs so that there is a relatively small amount of “silent” time available for the weaker hidden STA to retry its uplink transmissions. The hidden node problem may also occur on the downlink when downlink transmissions from two access points, which are hidden from each other, collide at an STA. Two techniques are conventionally used to improve performance in presence of interference: dynamic channel assignment (DCA) and the request-to-send/clear-to-send (RTS/CTS) protocol.
In a system that employs DCA, each access point is assigned one frequency from a set of available frequencies and uses this frequency to communicate with all of its associated STAs. Neighboring access points can be assigned different frequencies, e.g., using a tiling algorithm, to limit interference between transmissions associated with the different access points. The effectiveness of DCA depends on the availability of a sufficient number of channels so that different channels can be allocated to all of the neighboring access points. For example, the 2 GHz band is often preferred in Wi-Fi systems due to lower path loss, greater coverage and signal-to-noise ratios (SNRs), and because it is compatible with legacy STAs. However, the 2 GHz band only has 3 channels, which is frequently insufficient to provide each neighboring access point with a different channel. The 5 GHz band has twenty 20 MHz channels, but only a subset of these channels operate at the maximum FCC transmit power. Wi-Fi systems that operate according to the 802.11ac standards support three or four 80 MHz channels, assuming full access to dynamic frequency selection (DFS) enabled bands. WiFi service providers (WISP) may offer a fixed frequency plan, in which case changing channels would cause interference to a neighboring cell. In these cases (e.g., 2 GHz usage, 802.11ac, and outdoor APs using high power, fixed frequency plan), DCA may not be able to assign a free channel to each access point. Furthermore, although DCA helps with external interference caused by transmissions associated with other access points, DCA does nothing to improve fairness between hidden STAs associated with the same access point.
The RTS/CTS protocol attempts to reduce collisions by allowing STAs to send an RTS frame that indicates that the STA would like to transmit information over the uplink to the access point. The access point replies with a CTS frame that indicates that the requesting STA is free to transmit information for a time interval. Other STAs that detect the CTS frame are to refrain from transmitting during the time interval indicated in the CTS frame. The RTS/CTS protocol is not completely effective in preventing collisions because there is a vulnerability period between the time that the RTS is transmitted and the time that the CTS is heard and decoded at the (potentially interfering) other STAs, which can legally transmit during the vulnerability period. Moreover, hidden STAs may not detect the RTS and may therefore continue to transmit until the hidden STA detects the CTS. Transmissions from the hidden STAs can collide with transmissions from the requesting STA during the vulnerability period. The likelihood of collisions increases as the channel utilization increases and, consequently, relatively strong STAs garner an increasing share of the available resources at higher channel utilizations. At very high channel utilizations, the strong STAs can completely monopolize the available resources and shut out the weaker STAs.